Charge Transfer Interaction of Intermolecular Hydrogen Bonds in 7-Azaindole(MeOH)n (n = 1, 2) with IR-Dip Spectroscopy and Natural Bond Orbital Analysis

Abstract

Resonance-enhanced two-photon ionization (RE2PI) spectra of the deuterated 7-azaindole [7AI](MeOH)(n) (n = 1-3) clusters in the 0-0 region of the S(1)-S(0) (pi pi*) transition and IR-UV ion-dip spectra of the deuterated 7AI(MeOH)(n) (n = 1, 2) in the NH and OH stretch regions are observed in the gas phase to investigate the effect of charge transfer delocalization interaction on intermolecular hydrogen bonds. Two and three isotopomers are identified for 7AI(MeOH)(1)-d(1) and 7AI(MeOH)(2)-d(2), respectively, where 7AI(MeOH)(1)-d(1) has a deuterium atom in the NH or OH group and 7AI(MeOH)(2)-d(2) has two deuterium atoms in the NH and OH groups or in the two OH groups. The local modes of the NH and OH groups are successfully observed in the IR-dip spectra upon deuteration. The NH and OH stretch fundamentals of the 7AI(MeOH)(1)-d(1) and 7AI(MeOH)(2)-d(2) clusters are red-shifted from the corresponding ones of the 7AI and MeOH monomers. The observed red-shifts in 7AI(MeOH)(1)-d(1) and 7AI(MeOH)(2)-d(2) are correlated with the second-order perturbative energies obtained by the natural bond orbital analysis, suggesting that the charge transfer delocalization interaction plays an important role in stabilizing the intermolecular hydrogen bonds in 7AI(MeOH)(n) (n = 1, 2).